Research Professor of Psychology, University of North Carolina at Chapel Hill. Lecturer in Psychology, Duke University, Durham, North Carolina. Author of Human Motivation: A Guide to Information Sources and others.

forces acting either on or within a person to initiate behaviour. The word is derived from the Latin term motivus (“a moving cause”), which suggests the activating properties of the processes involved in psychological motivation. Psychologists study motivational forces to help explain observed changes in behaviour that occur in an individual. Thus, for example, the observation that a person is increasingly likely to open the refrigerator door to look for food as the number of hours since the last meal increases can be understood by invoking the concept of motivation. As the above example suggests, motivation is not typically measured directly but rather inferred as the result of behavioral changes in reaction to internal or external stimuli. It is also important to understand that motivation is primarily a performance variable. That is, the effects of changes in motivation are often temporary. An individual, highly motivated to perform a particular task because of a motivational...